A. Field of the Invention
The present invention relates to a method and device for determining pulse code modulation (PCM) codeword translations imposed on signals used in a PCM data communication system. The communication system of particular interest herein uses the public digital telephone network (DTN) to transmit data. The presence of Robbed-Bit-Signaling (RBS) and/or a Network Digital Attenuators (NDA) within the DTN impacts negatively upon the communication system performance. Determining the presence of translations in the system allows the communication devices to minimize the impact of the translations and fully utilize the available PCM codewords.
B. Description of the Related Art
For many years the public digital telephone network (DTN) has been used for data transmission between modems. Typically, a modulated carrier is sent over a local loop to a service provider (e.g., a Regional Bell Operating Company), whereupon the service provider quantizes the signal for transmission through the DTN. A service provider that is located near the receiving location converts the digital signal back to an analog signal for transmission over a local loop to the receiving modem. This system is limited in the maximum achievable data rate at least in part by the sampling rate of the quantizers, which is typically 8 kHz (which rate is also the corresponding channel transmission rate, or clock rate, of the DTN).
Furthermore, the analog-to-digital (A/D) and digital-to-analog (D/A) conversions are typically performed in accordance with a non-linear quantizing rule. In North America, this conversion rule is known as xcexc-law. A similar non-linear sampling technique known as A-law is used in certain areas of the world such as Europe. The nonlinear A/D and D/A conversion is generally performed by a codec (coder/decoder) device located at the interfaces between the DTN and local loops. Alternatively, these devices are referred to herein as a DAC (digital-to-analog converter) and an ADC (analog-to-digital converter).
It has been recognized that a data distribution system using the public telephone network can overcome certain aspects of the aforesaid limitations by providing a digital data source connected directly to the DTN, without an intervening codec. In such a system, the telephone network routes digital signals from the data source to a client""s local subscriber loop without any intermediary analog facilities, such that the only analog portion of the link from the data source to the client is the client""s local loop (plus the associated analog electronics at both ends of the loop). The only codec in the transmission path is the one at the DTN end of the client""s subscriber loop.
FIG. 1 shows a block diagram of a data distribution system. The system includes a data source 10, or server, having a direct digital connection 30 to a digital telephone network (DTN) 20. A client 40 is connected to the DTN 30 by a subscriber loop 50 that is typically a two-wire, or twisted-pair, cable. The DTN routes digital signals from the data source 10 to the client""s local subscriber loop 50 without any intermediary analog facilities such that the only analog portion of the link from the server 10 to the client 40 is the subscriber loop 50. The analog portion thus includes the channel characteristics of the subscriber loop 50 plus the associated analog electronics at both ends of the subscriber loop 50. The analog electronics are well known to those skilled in the art and typically include a subscriber line interface card at the central office that includes a codec, as well as circuitry used to generate and interpret call progress signals (ring voltage, on-hook and off-hook detection, etc.). In the system of FIG. 1, the only codec in the transmission path from the server 10 to the client 40 is a DAC located at the DTN 20 end of the subscriber loop 50. It is understood that the client-side, or subscriber-side, equipment may incorporate an ADC and DAC for its internal signal processing, as is typical of present day modem devices. For the reverse channel, the only ADC converter in the path from the client 40 to the server 10 is also at the DTN 20 end of the subscriber loop 50.
In the system of FIG. 1, the server 10, having direct digital access to the DTN 20 may be a single computer, or may include a communications hub that provides digital access to a number of computers or processing units. Such a hub/server is disclosed in U.S. Pat. No. 5,528,595, issued Jun. 18, 1996, the contents of which are incorporated herein by reference. Another hub/server configuration is disclosed in U.S. Pat. No. 5,577,105, issued Nov. 19, 1996, the contents of which are also incorporated herein by reference.
In the system shown in FIG. 1, digital data can be input to the DTN 20 as 8-bit bytes (octets) at the 8 kHz clock rate of the DTN. This is commonly referred to as a DS-0 signal format. At the interface between the DTN 20 and the subscriber loop 50, the DTN 20 codec converts each byte to one of 255 analog voltage levels (two different octets each represent 0 volts) that are sent over the subscriber loop 50 and received by a decoder at the client""s location. The last leg of this system, i.e., the local loop 50 from the network codec to the client 40, may be viewed as a type of baseband data transmission system because no carrier is being modulated in the transmission of the data. The baseband signal set contains the positive and negative voltage pulses output by the codec in response to the binary octets sent over the DTN. The client 40, as shown in FIG. 1, may be referred to herein as a PCM modem.
FIG. 3 shows a xcexc-law to linear conversion graph for one-half of the xcexc-law codeword set used by the DTN 20 codec. This conversion is fully defined in ITU-T Recommendation G.711 (1988), Pulse Code Modulation (PCM) of Voice Frequencies, the contents of which are hereby incorporated herein by reference. As shown in FIG. 3, the analog voltages corresponding to the quantization levels are non-uniformly spaced and follow a generally logarithmic curve. It should be noted that the analog voltages are represented in FIG. 3 as decimal values based on a 16 bit conversion. This is only for illustrative purposes, and 12 bits may be used as set forth in G.711. In other words, the increment in the analog voltage level produced from one codeword to the next is not linear, but depends on the mapping as shown in FIG. 3, and Recommendation G.711. Note that the vertical scale of FIG. 3 is calibrated in integers from 0 to 32,124. These numbers correspond to a linear 16-bit A/D converter. As is known to those of ordinary skill in the art, the sixteenth bit is a sign bit which provides integers from 0 to xe2x88x9232124 which correspond to octets from 0 to 127, not shown in FIG. 3. Thus FIG. 3 can be viewed as a conversion between the logarithmic binary data and the corresponding linear 16-bit binary data. It can also be seen in FIG. 3 that the logarithmic function of the standard conversion format is approximated by a series of 8 linear segments.
The conversion from octet to analog voltage (or a digital representation of the analog voltage, as discussed above) is well known, and as stated above, is based on a system called xcexc-law coding in North America and A-law coding in Europe. Theoretically, there are 256 points represented by the 256 possible octets, or xcexc-law codewords. The format of the xcexc-law codewords is shown in FIG. 2, where the most significant bit b7 indicates the sign, the three bits b6-b4 represent the linear segment, and the four bits, b0-b3 indicate the step along the particular linear segment. These points are symmetric about zero; i.e., there are 128 positive and 128 negative levels, including two encodings of zero. Since there are 254 non-zero points, the maximum number of bits that can be sent per signaling interval (symbol) is just under 8 bits. A xcexc-law or A-law codeword may be referred to herein as a PCM codeword. It is actually the PCM codeword that results in the DTN 20 codec to output a particular analog voltage. The codeword and the corresponding voltage may be referred to herein as xe2x80x9cpoints.xe2x80x9d
Other factors, such as robbed-bit signaling, digital attenuation (pads), channel distortion and noise introduced by the subscriber loop, and the crowding of points at the smaller voltage amplitudes and the associated difficulty in distinguishing between them at the decoder/receiver, may reduce the maximum attainable bit rate. Robbed Bit Signaling (RBS) involves the periodic use of the least significant bit (LSB) of the PCM codeword by the DTN 20 to convey control information. Usually the robbed bit is replaced with a logical xe2x80x981xe2x80x99 before transmission to the client 40. The DTN may also replace the LSB alternately with a xe2x80x981xe2x80x99, and then a xe2x80x980xe2x80x99. Additionally, in an RBS interval, a codec may produce an analog voltage level that is between levels corresponding to valid codewords. This is referred to herein as xc2xd bit RBS.
The DTN performs RBS on a cyclic basis, robbing the LSB of an individual channel every sixth PCM codeword. In addition, due to the fact that a channel might traverse several digital networks before arriving at the terminus of the DTN 20, more than one PCM codeword per 6 time slots could have a bit robbed by the network in the case where each network link robbing a different LSB. Of course, at most only one time slot may suffer from xc2xd bit RBS.
To control power levels, some networks impose digital attenuators that act on the PCM codewords to convert them to different values. Unlike most analog attenuators, a network digital attenuator (NDA) is not linear. Because there is a finite number of digital levels to choose from, the NDA will be unable to convert each codeword to a unique, lower (or higher) magnitude codeword. The analog level ultimately transmitted by the codec over the subscriber loop 50 corresponds to the translated codeword.
RBS, xc2xd bit RBS, and NDAs can coexist in many combinations. For example, a PCM interval could have a robbed bit of type xe2x80x981xe2x80x99, followed by an NDA followed by a xc2xd bit RBS link.
It is evident that the above-described data transmission system may alter the points that are used to transmit data through the system: e.g., LSBs may be robbed during some time slots thereby making some points unavailable in that time slot; digital attenuators may make some points ambiguous; the codec may not generate the analog voltages accurately; and, noise on the local loop may prevent the use of closely spaced points for a desired error rate. Thus it is desirable to be able to determine which codewords are usable in spite of the PCM codeword translations, so that the communication devices may operate efficiently.
The present invention provides a method and apparatus for determining the characteristic response of a communication channel that utilizes the public Digital Telephone Network (DTN). In a communication system having a transmitter that supplies digital information directly to the DTN in the form of PCM codewords, and having a receiver connected to the DTN via an analog loop, any PCM codeword translation imposed by the DTN must be accounted for. Specifically, the translations imposed on PCM codewords traversing such a system must be detected or errors will result. The channel includes the DTN, which may have Network Digital Attenuators (NDA) and/or Robbed Bit Signalling (RBS), and a Digital-to-Analog Converter (DAC), (also known as a codec), as well as the analog characteristics of the local loop, typically a twisted pair of copper wires. The translation detection is especially useful in so-called PCM modulation schemes that utilize the DTN, where knowledge of codeword translations predicates the selection of available PCM codes used to represent data. This information is also useful when the data receiver, or PCM modem, makes determinations of which codes were actually sent by a transmitter, thus resolving the ambiguities imposed by PCM codeword translation.
Once the translation effects have been accounted for, the decision regions with the receiver""s quantizer may be updated to account for the characteristics of the particular codec in use on the analog subscriber loop by the DTN.